State four factors to study in research on agronomic characters of maize hybrid?

1. Yield and yield components: Grain yield (kg/ha), number of kernels per ear, ear length, and row number.
2. Plant morphology: Plant height, ear height, leaf area index, tassel length, and tassel branch number.
3. Physiological traits: Photosynthesis rate, transpiration rate, water use efficiency, and nutrient uptake.
4. Grain quality: Kernel weight, kernel hardness, protein content, starch content, and oil content.